A non standard well control method is to attempt to bring the well under control by circulating heavy mud to overbalance the surface pressures. The theory behind this method is that the well can then be opened and the string run deeper. This method is not reliable since there is no proper control of bottom hole pressures during circulation, nor during running the string back to bottom.
The well may kick again due to the loss of hydrostatic head caused by the following phenomena:
- While running pipe in the hole, light mud displaced by the drillstring will in turn displace heavy mud out of the hole.
- The influx height increases considerably when the string enters the influx.
- Gas migration results in expansion of the influx.
If the well has been brought temporarily under control with the string a considerable distance off bottom, it will be necessary to reduce the mud density gradually to a value near its original one, by running in and circulating in stages to bottom. Again, the method is not recommended since bottom hole pressures cannot be controlled properly and the position of the influx is unknown.
One way to overcome the possibility of loosing the overbalance whilst running back to bottom is to circulate the well to a higher mud weight which is sufficient to compensate for the following (hydrostatic) pressures after the string is stripped back to bottom:
- The annular pressure (Pa).
- The loss in hydrostatic head due to the influx volume increase during stripping the string back to bottom, assuming the influx to be around the smallest annular area. Take the migration rate and the time to strip back to bottom into account to estimate the influx volume.
- The effect of loss of hydrostatic head due to heavy mud returns which is caused by stripping the string back to bottom.
This method can only be used if the initial (maximum) overbalance, which occurs after the hole is circulated to heavy mud and prior to stripping back to bottom, is acceptable.
Other well control methods are:
- The volumetric method which should be immediately implemented until decisions are made on how to restore primary well control.
- The combined stripping and volumetric method whereby the bit is run back to bottom under controlled conditions after which the influx is circulated out.
- Bullheading.